Granted, I regret opening the thread now, for sheer surprise, but in all honesty, after watching today's episode, I HAD planned on typing that I think it was Pretty. The blackmailer just HAD to be a Crane to know about all the things that he/she did. Also, to have a lair with actual IMAGES of the he/she painted on the walls, on the Crane estate, was something that only a priveleged freak would have. A regular freak would just have a lair, with blank walls.

And the fact that he/she tortured Fancy before anyone, and the great lengths that they used to specifically target Fancy, also is a great indicator. Targeting Luis was great too since that was Fancy's object of affection.